Well MTT there are many players, and a very low chance of cashing big, so you have to set yourself up to make those big cashes, which means putting your stack at risk, so your chips are worth less essentially until you reach the final table, at which point it basically becomes a SNG.
SNG is all about making money, and cashing at a certain rate. The thing about SNG is that you can make the money by folding everyhand, so theres no need to make moves, tighter play works early, and your chips overall are worth much more.
I highly suggest reading colin moshmans book, i cant for the life of me remember what it's called but it should be easy to find, read that and it will explain the inner workings of ICM and how chip values differ from MTT and SNG.